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Re: Design for a small motor boat?

You looking to build, or buy? Say some more. Are you hoping to row this boat with relative ease? There are a lot of good little, lightly-powered, skiffs out there. A favorite of mine, though it couldn't be construed as a row boat, is the mid to late fifties Lyman Islander. It grew a bit, from 17 to 18. A similar form, but not lapstrake, is some of Harry Bryan's skiffs, if you want a more modern iteration.

The Lymans, I can attest to this because I did a bit of work on them, were very nice creations. A four cylinder gas Gray, which pushed the bote onto a plane with just enough power to make it work efficiently. It got going maybe seventeen knots. Yet just at the balance so it was fairly stingy with gas. Incredible materials. Old growth fir ply over white oak frames. Small dex fore and aft of good mahogany ply. I've sung their virtues before.

Re: Design for a small motor boat?

You'll be hard pressed to find a boat with an inboard in that size range, and most will be very old plans. Damn few small inboards being made these days -- everyone puts an outboard on the transom or in a well.

Big Question = WHY an inboard?

And DON'T take a pulling/rowboat design and try to convert it for inboard power -=- hull shape and many other elements are dead wrong for it.

There are lots of plans that meet your requirements without you having to learn some expensive and frustrating lessons about marine design the very hard way...

Re: Design for a small motor boat?

I'll be interested in hearing other people's comments on this. As to actually putting a (very) small inboard in an Hv 16 hull, in terms of the center of gravity, it would have to replace the centerboard trunk towards its aft end. Somewhat more robust parameters is probably what you want - you really have to define your use of the boat - but there is no reason that a small inboard launch cannot have some of the aesthetics that you enjoy with the 16. The HV 16 is fine ended, though with good reserve bouyancy and strong haunches, for rowing and sailing - weight distribution is important, either evenly distributed through, or focused in the middle of the boat. She will squat with one person perched on the transom or with much thrust coming from inboard propulsion. With the addition of an inboard, displacement - ie weight and capacity, as well as trim is most important. Poulsbo Boats look like small tugboats, but they are really very trim sensitive and easily overpowered. You can also bat this around with me on the PM board. Nevertheless, good to open the question up to forumites.

Re: Design for a small motor boat?

The Handy Billy, though a bit longer is a very handsome boat. Something along those lines could be incorporated into a smaller boat.

I like the idea of a small diesel propelling a small displacement craft, but part of the problem as I see it is that the placement of the motor is right smack in the middle of the boat, greatly limiting your usable space.

Re: Design for a small motor boat?

An entirely different kind of launch - the Disappearing Propeller Boat, AKA Dippy or Dispro. An overgrown double ended pulling boat with inboard and exotic propshaft arrangement. Somewhat long and narrow, maybe 17' at the minimum, probably not what you had in mind.

Re: Design for a small motor boat?

aklimo,
you might want to keep in mind the noise factor with an inboard (especially a diesel) in a small boat. A lightweight lapstrake hull will act as a sound board verses a heavily constructed hull with a lot of mass to absorb some of the sound. Unless you power it with a tucky-tucky antique engine you will need lots of sound deadning insulation.

When the last tree is cut
When the last river is dry
When the last fish is caught
Only then will Man realize that he cannot eat money.

Re: Design for a small motor boat?

Since you value aesthetics highly you might consider electric power as rbgarr suggested. No noise is a great feature for plesant cruising and the motor itself is so small you can fit it in without intruding into the boat's interior space nearly as much as an ICE. The batteries can go out-of-sight under the seats and/or fore and aft decks, wherever their weight keeps the boat floating level.

The negative of electric power is high weight and short range. In a 15' wl boat you would be limited to displacement speeds (under 6 mph) and range with a reasonable battery bank (4 12v 100 ah, 250# total) would be about 3 or 4 hours at 4 or 5 mph.

If the range and speed constraints are acceptable it would be a good solution. You also have to be able to plug it in to recharge. Would not work on a mooring but no problem for a boat on a trailer in your garage.

Small gasoline marine inboard motors are non-existant. An air cooled lawnmower type engine is inexpensive but awfully noisy. A water cooled marine diesel is a good functional solution but still noisy and quite expensive. If I were you and couldn't live with electric's constraints I'd find a design with an outboard hidden in a well. There are lapstrake sailboats (Oughtred, Selway-Fisher, Wellsford and other designers) that could be built with out the rig and a motor well. A modern 4-stroke outboard is very quiet and if you are OK going slow you only need about 5 hp.

Re: Design for a small motor boat?

Another problem with lawnmower engines is that they are usually vertical shaft (but lawn tractors may be different?). And most garden engines are air cooled, so you have the noise problem. How hard would it be to convert an engine to water cooling? I guess you could solder a water jacket around the cylinder head and run water through that. But you still have the problem of fitting the water pump to the engine...

Re: Design for a small motor boat?

Originally Posted by BarnacleGrim

Another problem with lawnmower engines is that they are usually vertical shaft (but lawn tractors may be different?). And most garden engines are air cooled, so you have the noise problem. How hard would it be to convert an engine to water cooling? I guess you could solder a water jacket around the cylinder head and run water through that. But you still have the problem of fitting the water pump to the engine...

What about scooter engines?

The big problem with any small displacement engine is that it must operate in a specific RPM band to generate enough torque to carry a load. Usually that RPM range is too high for driving a boat propeller. Some type of gear or vee belt system is needed to get the prop shaft spinning slow enough. Gear boxes can be expensive and belt drives require space and must be covered for safety. There have been builders here who have built their own speed reducers but isn't something that can be quickly put together.
If you can find a suitable small implement motor with a built in gear reduction you are half-way there but new motors aren't all that cheap.
Many small air-cooled motors are die cast aluminum. You certainly can't solder on them and welding on such alloys, if it can be done, would require someone with a high skill level.

Re: Design for a small motor boat?

Originally Posted by mcdenny

Small gasoline marine inboard motors are non-existant. An air cooled lawnmower type engine is inexpensive but awfully noisy. A water cooled marine diesel is a good functional solution but still noisy and quite expensive. If I were you and couldn't live with electric's constraints I'd find a design with an outboard hidden in a well. There are lapstrake sailboats (Oughtred, Selway-Fisher, Wellsford and other designers) that could be built with out the rig and a motor well. A modern 4-stroke outboard is very quiet and if you are OK going slow you only need about 5 hp.

All very good points though I must disagree with the small gas engine thing: Kohler's Aegis line of liquid cooled motors are available from 20-30 Hp and are remarkably quiet, and stingy on fuel; something on the order of 1.3 GPH for the smallest size. So if you have a boat that can run 18 Kt, you're getting something like 20 statute MPG from a boat, which is spectacular.

The Atkins' boats are big favorites here at WBF. If your size limit were a little bigger I'd suggest looking at Sergeant Faunce but maybe that's my own little peccadillo.